Air filter



FIG. 1 is a front perspective view of an air filter showing our new design;

FIG. 2 is a front view thereof;

FIG. 3 is a left side view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a top view thereof;

FIG. 6 is a bottom view thereof; and,

FIG. 7 is a rear view thereof.

The broken lines depicting the remainder of the air filter form no part of the claimed design. 

CLAIM The ornamental design for an air filter, as shown and described. 